Opportunities can often be found from trawling through statistics, this is how markets are made after all. Whilst some markets are very accurate and have huge volumes of cash going through them others can be more illiquid and these can often offer opportunities for the well clued up Bookie Basher.
Plenty of football data is available for free at football-data.co.uk.
WhoScored.com is great for stats on individual players such as cards received, goals outside the box etc.
Soccer Stats have some very useful data in an easy to read format http://www.soccerstats.com/latest.asp?league=england
538 do some decent analysis of football data which can be found here https://projects.fivethirtyeight.com/soccer-predictions/premier-league/
The Dixon and Coles model was one of the first proper studies in to football betting.
The racingpost is a good free resource for data if you want to analyse the horses.
Covers have a load of information on American sports https://www.covers.com/
538 specialise in this area see https://projects.fivethirtyeight.com/2017-nfl-predictions/games/
There is an interesting collection of data at Tennis Abstract here http://www.tennisabstract.com/